1. Field of the Disclosure
The present disclosure relates to a method for discharging a hydrogen storage system, which is present in the annular space of a receiver tube, wherein the annular space is formed between an outer-lying tubular jacket and an inner-lying absorber tube of the receiver tube, and the outer-lying tubular jacket is joined to the absorber tube via a wall in a gas-tight manner. The wall is generally composed of metal and includes a glass-metal transition element and an expansion compensating element as well as other connection elements. Furthermore, the disclosure relates to a device for discharging a hydrogen storage system that is present in the annular space of the receiver tube.
2. Background of the Disclosure
Solar collectors comprise a collector mirror—for example, a parabolic cylindrical mirror (parabolic trough)—and a receiver tube, and are utilized in solar thermal power plants preferably for the generation of electricity. The receiver tube is arranged in the focal line of each collector mirror and is generally composed of an absorber tube made of steel, which has a radiation-absorbing layer, and a tubular jacket made of glass, which surrounds the absorber tube and thermally insulates it. In the known solar thermal power plants, a thermal oil, which is utilized as a heat transfer medium, is carried through the absorber tube and can be heated to a temperature of about 400° C. by means of the solar radiation reflected from the collector mirrors and focused on the absorber tube. The energy stored in the heat transfer medium is released via a heat exchanger into a steam circuit and converted to electrical energy in a turbine.
An annular space is formed in the receiver tube between the absorber tube and the tubular jacket. This annular space serves to minimize heat losses at the outer surface of the absorber tube and thereby to increase the efficiency of the solar collector. For this purpose, the annular space is evacuated, so that heat can be emitted from the absorber tube to the greatest extent possible only in the form of radiation.
The thermal oil utilized as the heat transfer medium in the absorber tube exhibits a temperature-dependent aging and a decomposition rate associated therewith. The decomposition of the heat transfer medium in the long run leads to the formation of various decomposition products, which include, among others, hydrogen. The amount released during the aging process depends, on the one hand, on the thermal oil used and the operating conditions in the solar thermal power plants and, on the other hand, on the degree of purity of the heat transfer medium.
By way of permeation, the hydrogen released by decomposition of the heat transfer medium partially enters the evacuated annular space of the receiver. Because the hydrogen permeability of glass is less than that of steel by orders of magnitude, the hydrogen accumulates in the annular space. In consequence, the pressure in the annular space rises and the thermal conductivity of the annular space increases as well. This occurs until an equilibrium prevails between the partial pressures of hydrogen in the absorber tube and in the annular space. It is especially a drawback in this case that hydrogen has a higher thermal conductivity than air, for example, so that, as hydrogen permeation progresses further, the thermal conductivity in the annular space is even better than that of the air outside the receiver tube. In consequence, the efficiency of the receiver tube drops and hence so does that of the complete solar collector.
In order to counteract this increase in the partial pressure of hydrogen in the annular space and thereby maintain the high efficiency of the receiver tube, various solutions are known from the prior art.
Known from DE 10 2009 017 741 A1, for example, is a receiver tube, which comprises a valve arrangement that extends through the wall of the tubular jacket into the annular space. This valve arrangement makes it possible to flush the annular space or to evacuate it whenever a large proportion of interfering substances, such as hydrogen, for example, has accumulated.
In addition, the hydrogen that has diffused into the annular space can be bound by means of a getter. However, the absorption capacity of such materials is limited, so that, once a material-specific maximum absorption capacity has been attained, no further hydrogen can be bound and the pressure in the annular space increases once again.
Receiver tubes with a getter material arranged in the annular space are known from WO 2004/063640 A1, for example. In the device described in this specification, the getter material is arranged in getter bridges between the absorber tube and the tubular jacket directly in the annular space. The getter bridge produces a spacing between the absorber tube and the getter, so that the thermal load on the getter is reduced and its absorption capacity is thereby improved. However, apart from the use of a getter material, no other solution for diminishing the hydrogen concentration in the annular space has been provided, so that the drawbacks of the getter described above still remain.
In order to alleviate the problem of getter materials, DE 198 21 137 A1 discloses a receiver tube for solar thermal applications, in which, in addition, noble gas with a partial pressure of up to several hundred mbars is present in the annular space. The advantage of this solution is that many noble gases have a lower thermal conductivity than air, so that the thermal conduction through the annular space and the deterioration in efficiency associated therewith can be reduced. However, the drawback of this design is that the annular space is filled with noble gas from the very start, so that, already directly after installation of the solar collector, a lower efficiency is achieved than for the case of an evacuated annular space.
Alternative embodiments, such as, for example, those disclosed in DE 10 2005 057 276 B3, provide for at least one gas-tight sealed tank, filled with at least one noble gas, in the annular space, from which the noble gas is admitted to the annular space once the getter material is exhausted. The drawback of this alternative embodiment is that the solar collector and, in particular, the receiver tube must be fabricated already with a filled tank. Retrofitting is not possible, so that the customer needs to make a decision directly during fabrication of the receiver tube about bearing the extra costs and the increased work effort involved. Another difficulty is presented in opening the tank, which can occur only with increased effort.
A method for opening the tank and for filling the annular space with noble gas is known from DE 10 2011 082 772 B9, wherein the tank is opened by means of a laser drilling method. A laser beam is directed from the outside through the tubular jacket onto the tank, which is irradiated until an opening forms in the tank and the protective gas is released. However, a drawback of this disclosure is also that retrofitting of the receiver tube with the protective gas tank is not possible and the customer needs to bear the increased costs and fabrication effort already during the fabrication, even though the noble gas is employed only a long time after startup.
Thus, at present, no method is known for making it possible to restore in a satisfactory manner the efficiency of a receiver tube that has already suffered losses in performance due to elevated hydrogen pressure in the annular space.